The present invention is the use of a specific catalyst composition for carbon oxide methanation reactions for fuel cells. Specifically, when a mixture of gases containing hydrogen, carbon dioxide, carbon monoxide, and water is passed over the catalyst in a reaction zone having a temperature below the temperature at which the shift reaction occurs and above the temperature at which the selective methanation of carbon monoxide occurs, the catalyst efficiently facilitates the selective hydrogenation of carbon monoxide using H2 that is present in the reformate and reduces the concentration of the CO to levels equal to or less than about 50 ppm and demonstrates a carbon monoxide (CO) methanation selectivity of greater than about 50%. This is a significant improvement over selective methanation catalysts of the prior art.
In a fuel cell, such as a Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack, chemical energy of a fuel is converted into electrical energy. Typically, the fuel used is a hydrogen rich gas supplied to the fuel cell by a fuel processor. However, the gas from the fuel processor may further comprise unconverted hydrocarbon, water, carbon dioxide and carbon monoxide. The carbon monoxide, in particular, is detrimental to the PEMFC stack because the carbon monoxide can poison the noble metal electrodes utilized by the fuel cells, thereby reducing the electrical output.
Preferably, the CO concentration for a fuel cell feed should be at a level below about 100 ppm, and more preferably to a level of less than about 50 ppm. However, as received from the fuel processor, the CO concentrations may be in excess of about 1 wt %, thus requiring further reduction of CO concentration. Some typical methods for reducing the CO concentration include selective catalytic oxidation of CO, pressure swing adsorption, hydrogen separation by membrane, and selective methanation of CO.
Selective catalytic oxidation of CO (Eq. 1) is a well-known process for reducing the CO concentration for fuel cells. But, oxidation of hydrogen (Eq. 2) is a competitive reaction.½O2+CO→CO2  Eq. 1½O2+H2→H2O  Eq. 2Thus, in order to maximize the concentration of hydrogen gas and minimize the concentration of carbon monoxide, it is necessary to have reaction conditions wherein Eq. 1 is favored over Eq. 2. One option for achieving this is to have a highly specific catalyst for the oxidation of carbon monoxide and to limit the oxygen concentration so that the oxygen is consumed primarily for the production of carbon dioxide. Theoretically, this is achievable, but in practice there are wide swings in the CO concentrations produced by the fuel processor and it can be difficult to adjust the oxygen input to track the CO concentration. Because the CO is more detrimental to the fuel cell than water, it is typical for excess oxygen to be fed into the reactor thereby essentially ensuring that the CO will be converted to CO2. The disadvantage is that significant quantities of H2 are converted to water by operating in this manner.
Pressure swing adsorption is an industrially proven technology, but it requires relatively high pressure operation. Thus, while this process may be effective for use in larger fuel cells, it is not practical at this time for smaller fuel cells.
Hydrogen separation by membrane is effective for separating hydrogen from carbon monoxide. But the process requires a substantial pressure drop to effect the separation, and the cost and durability of the membranes still must be proven.
Selective methanation (Eq. 3) is a process whereby carbon monoxide is reacted with hydrogen in the presence of a catalyst to produce methane and water and methanation of carbon dioxide is minimized. Commonly used in ammonia plants, total carbon oxide methanation is known to reduce carbon monoxide and carbon dioxide concentrations to levels as low as about 5 ppmv to 10 ppmv, and the industrial catalysts are not selective. However, in most fuel cell applications, the selective methanation reaction is accompanied by a reverse water-gas-shift reaction (Eq. 4), which also is generally facilitated by a catalyst. Thus, while the CO concentration is being reduced through methanation, additional carbon monoxide is formed from the carbon dioxide present to maintain the equilibrium of the water-gas-shift reaction.CO+3H2→CH4+H2O  Eq. 3CO2+H2CO+H2O  Eq. 4Under the proper reaction conditions and with a non-selective methanation catalyst, the CO2 may be methanated as shown in Eq. 5.CO2+4H2→CH4+2H2O  Eq. 5But, this is generally an undesirable reaction because it further consumes H2 and the CO2 methanation is normally accompanied by a temperature rise in the reactor that can lead to “run-away” conditions. Considering that the carbon dioxide concentration is greater than 10 times that of carbon monoxide, achieving selectivity is not thermodynamically favorable. Thus, it would be advantageous to have a catalyst that is highly selective for CO methanation, essentially suppresses CO2 methanation and does not facilitate the conversion of CO2 to CO through the water-gas-shift reaction.
In the prior art methanation processes, precious metals supported on non-zeolitic materials, such as Al2O3, SiO2, and TiO2, have been used as catalysts in the selective methanation of CO (see, for example, U.S. Pat. No. 3,615,164 and U.S. Pat. Pub. No. 2003/0086866). For example, in WO 01/64337, ruthenium (Ru) on a carrier base support of Al2O3, TiO2, SiO2, ZrO2, or Al2O3—TiO2 with egg-shell structure is taught to reduce the CO to concentrations of about 800 ppm with 70-80% selectivity under an atmosphere of CO at 0.6%, CO2 at 15%, H2 at 64.4%, H2O at 20% and GHSV=10,000 H−1. However, for an efficient PEMFC power system, the CO concentration should be less than about 100 ppm, and preferably equal to or less than about 50 ppm. Since the CO concentration from the selective methanation processes using the prior art catalysts are significantly higher than the desired maximum concentration for a PEMFC stack, these catalysts cannot be practically used in PEMFC power systems.
Thus, it would be advantageous to have a catalyst that is highly selective for CO methanation, essentially suppresses CO2 methanation and does not facilitate the conversion of CO2 to CO through the water-gas-shift reaction.